This invention relates to a process for the preferential alkylation or acylation of a meta-disubstituted benzene.
Processes for the preparation of meta-disubstituted benzenes, particularly meta-dichlorobenzene, are known in the art. Unfortunately, since very little meta-dichlorobenzene is produced by the direct chlorination of benzene, meta-dichlorobenzene is commonly prepared by indirect routes, e.g. from meta-dinitrobenzene or meta-chloronitrobenzene. A process often used is a complex one involving the reduction of nitro groups to amino groups followed by diazotization and treatment of the diazonium salt with cuprous chloride. Meta-dichlorobenzene produced this way is expensive primarily because product yield in the multi-step process is low.
Recently, a method for producing meta-dichlorobenzene from benzene was disclosed in U.S. Pat. No. 3,358,046 to Offenhauer and Rodewald. In the practice of this method, the benzene ring is first alkylated and then chlorinated to form an ar,ar-dichloro-ar-alkylbenzene in which the chloro groups are meta to each other. The resulting compounds are then dealkylated to give the desired meta-dichlorobenzene. Unfortunately, catalyst requirements for the alkylation step are generally excessive, e.g., on the order of about 25 weight percent based on the aromatic substrate. Also, it is generally necessary to place at least two and often three alkyl groups on each aromatic ring to be chlorinated in order to obtain the desired meta-substitution of the chloro groups. Accordingly the demand for alkylating agent is also excessive.
In view of the several undesirable aspects of the aforementioned prior art processes, it would be highly desirable to provide a process for making meta-dihalobenzenes requiring lesser amounts of aluminum trichloride catalyst and alkylating agent. More broadly, it would be desirable to provide a process for making meta-disubstituted benzenes and for preferentially alkylating or acylating such meta-disubstituted benzenes.